Today in San Francisco, Apple unveiled a new relationship with Twitter, where Twitter sign-in and sharing features will be built right into Apple's iPhone and iPad operating system. (A very rare thing for Apple, which is hesitant to rely on third party services for anything.)

Sounds like the sort of feature that would be just as good -- if not better -- with Facebook, which has many millions more users than Twitter? Indeed. But Apple had nothing to say about Facebook today.

While Apple has been able to integrate Facebook into some minor products, such as iPhoto, for whatever reason, the companies haven't been able to figure out a deal for major mobile integration. Last September, for example, Apple had to remove Facebook support from Ping, a social network, at the last minute.

Several weeks later, Facebook CTO Bret Taylor told reporters in New York that he was "very confident" that Facebook and Apple would figure out a way to work together on Ping.

But, publicly at least, nothing seems to have changed since then. Meanwhile, today, Apple just announced a big partnership with Twitter, one of Facebook's biggest rivals.

This leaves the door open for Facebook and Apple to potentially work something out between now and the fall, when iOS 5 ships to consumers, so Apple can announce Twitter and Facebook integration for the final product. Facebook, after all, is built into many other phone platforms, including Microsoft's Windows Phone 7, some Android flavors, etc.

Even if Facebook doesn't need Apple, and Apple doesn't need Facebook, it would be better for all of their users if they worked together, after all.

Or maybe they just aren't going to play nicely together.

Mark Zuckerberg and Steve Jobs obviously both know that they run two of the most important tech companies in the world. They don't need each other if they can't come to terms they can agree on. And maybe that's going to keep them apart.

Either way, watching this relationship evolve is only going to get more interesting over the years.